A lipophilic substance with high affinity for central benzodiazepine receptors has been extracted from rat brain tissue. The lipophilic substance reduces gamma-aminobutyric acid-stimulated uptake of chloride in synaptoneurosomes. In preliminary studies the benzodiazepine antagonist, flumazepil, blocked the inhibitory effect of the lipophilic substance on gamma-aminobutyric acid- stimulated chloride uptake. The lipophilic substance does not appear to be a benzodiazepine, a beta-carboline or a polypeptide. The objectives of this grant should help explain why humans vary in response to the benzodiazepines. The first specific aim is to obtain the lipophilic substance in 95 to 100 purity. State of the art spectroscopic equipment will be used to determine the structure of the lipophilic substance. The affinity of the lipophilic substance for central and peripheral benzodiazepine receptors will be assessed by its ability to displace -H-flumazepil and -H-PK11195 specific binding, respectively. The selectivity of the lipophilic substance for benzodiazepine receptors will be determined by its ability to displace the specific binding of radio-ligands with selectivity for other receptors. The potency of the lipophilic compound in decreasing muscimol-stimulated chloride uptake in synaptoneurosomes will be measured. To control for stress induced alterations of chloride transport, a handled-habituated group will be included in these latter studies. The mechanism by which the lipophilic substance initiates the decrease in muscimol-stimulated chloride uptake will be clarified. Flumazepil, PK11195 and Ro15- 4513 will be examined for an ability to antagonize the reduction in muscimol-stimulated chloride uptake elicited by the lipophilic substance.